This invention relates to the field of amplitude modulated stereophonic transmission and, more particularly, to the minimization of spectrum spreading.
In the transmission of AM stereo signals, it is theoretically possible, under certain peculiar signal conditions, to produce small amounts of sideband signals which fall outside the desired channel. For this to happen, there must be a large amount of high frequency energy in the stereo or difference channel. With 10 kHz channel spacing, this is an almost impossible condition. With 9 kHz channels, there is a slightly greater chance of the condition occurring.
In another patent, No. 4,338,491, assigned to the assignee of the present invention, one method of reducing the possibility was disclosed. In the simplest form of that invention, the difference channel signal was coupled through a pair of filters, one low pass and one high pass. The output of the high pass filter was compressed before being added to the output of the low pass filter, to be used in a stereo encoder to modulate a carrier. In a second embodiment of the above-referenced invention, a more elaborate system for controlling spectrum spread was provided, but at the cost of much greater complexity in the circuit.